Currently, most of mobile communication networks are CS networks, such as the global system for mobile communications (GSM) and code division multiple access (CDMA) network. Operators establish perfect and rich service platforms based on the CS network, of which a mobile switching center (MSC) is responsible for call routing and service logic execution, for example, call forwarding service and call hold service.
The CS network needs supports from MSCs in visited locations to provide services, making it difficult to launch new services. The IMS is a packet-switched (PS)-based service network, and can provide services without supports from visited locations, making it convenient to launch new multimedia services. The IMS supports user equipment (UE) in connecting to an IMS network through various PS access networks, for example, general packet radio service (GPRS), to implement IMS multimedia services.
The PS-based IMS network represents the future development trend of communication networks. However, the IMS network is so complex that it cannot be deployed completely in a short term. In addition, CS network users may not replace traditional terminals with new IMS terminals within a short time. Thus, the CS network may coexist with the IMS network in the coming years. The CS network and the IMS network need to be operated at the same time.
During the evolution of the CS network to the IMS network, the Third Generation Partnership Project (3GPP) proposes ICS, requiring the UE to set up an IMS call by carrying voice or video phone (VP) media over the CS network, so as to weaken or remove the service processing logic of the MSC. The UE receives or transmits IMS call related control signalling through a new IMS CS control channel (ICCC). The ICCC may be set up over unstructured supplementary service data (USSD) and called ICCC-cs, or set up over PS and called ICCC-ps.
A calling process of setting up an ICS call through a CS network in the prior art includes the following steps:
Steps 101-104: A terminal device sets up an ICCC with an IMS CS control function (ICCF), and sends the telephone number of a called party to the ICCF through the ICCC. The details are as follows:
Step 101: The terminal device sends an INVITE message to the ICCF, where the INVITE message includes the telephone number of the called party. The INVITE message may be transmitted through the USSD or PS network.
It is understandable that the PS network may be a packet access network such as GPRS, but the specific transmission mode may be selected according to the network services that are available to the user terminal.
Step 102: The ICCF returns a provisional response message (183), where the provisional response message may include the ICCF number. The ICCF number may also be pre-configured in the terminal device. In this case, the ICCF number does not need to be returned.
Step 103: The terminal device sends a provisional response acknowledgement message (PRACK) to the ICCF.
Step 104: The ICCF returns a 200OK message to acknowledge the receipt of the PRACK message.
Step 105: The terminal device and the ICCF set up a CS call leg.
Step 106: The ICCF and the called party set up a second call leg.
Step 107: The ICCF sets up a call connection between the terminal device and the second party by binding the call leg set up between the ICCF and the terminal device and the second call leg set up between the ICCF and the second party.
The ICCF may be regarded as a back-to-back-user-agent (B2BUA), which is equivalent to two user agents (UAs) that are bound. The first call leg set up between the ICCF and the terminal device and the second call leg set up between the ICCF and the called party are mutually independent. When one call leg is removed, the other call leg may exist. The setup of the first call leg and the second call leg may be triggered by each other. During the setup, the signalling interaction, media connection, and call connection are implemented between the terminal device and the second party.
The called process of setting up an IMS call through a CS network in IMS CS mode is similar to the calling process, and will not be further described.
On the basis of the above basic calling process in the prior art, the terminal device may send a request for setting up a session with a third party to the ICCF through the ICCC.
After setting up the call leg with the third party, the ICCF may set up a call connection between the terminal device and the third party by binding the CS call leg and the third call leg.
In addition, the ICCF may transfer sessions between the terminal device and the second party and between the terminal device and the third party by receiving control signalling and reusing the CS call leg.
During the research and practice of the prior art, the inventor discovers the following problems:
The above technical solution may set up an IMS call through the CS network, and transfer sessions between the terminal device and the second party and between the terminal device and the third party. However, if the media type for the call is changed during the call, the call may be disconnected due to the change of the media type after session transfer. For example, in the above technical solution, if the media type for the call between the terminal device and the second party is changed, the media type for the CS call leg set up between the terminal device and the ICCF is different from the media type for the third call leg set up between the ICCF and the third party after session transfer. This may disconnect the call between the terminal device and the third party.